Apparatus to measure the period of an input signal



Sept. 20, 1966 P. s. BENGSTON APPARATUS TO MEASURE THE PERIOD OF ANINPUT SIGNAL 5 Sheets-Sheet 1 Original Filed March 4, 1960 1 l l l l i lI |||I| liillll III! |I|||| l 523 1 w moEmwzmo $1 o ommwniz rwmmop/E2253: 61w wzo P58 mm mmk l 5135 M mm NN 51 f W $4123 A mwnEIm NEE m5252mm t h FEOTEw I u 8 N F! I l i l I I I I I I I I I I l I l I I l I II I |||l.

N m 0 T NS G v m m B S R ATTORNEY 5 Sheets-Sheet 2 I ATTY.

INVENTOR. P. S. BENGSTON BY N s. BENGSTON APPARATUS TO MEASURE THEPERIOD OF AN INPUT SIGNAL VIL FIGS.

Sept. 20, 1966 Original Filed March 4, 1960 P 20, 1966 P. s. BENGSTON3,274,500

APPARATUS TO MEASURE THE PERIOD OF AN INPUT SIGNAL Original Filed March4, 1960 5 Sheets-Sheet 5 F 16.4 b. l

C l43x FICA. I I I I I I i I m i i I49 i i FlG.4h.

INVENTOR.

P. S. BENGSTON ATTY.

United States Patent Office 3,274,500 Patented Sept. 20, 1966 3,274,500APPARATUS T MEASURE THE PERIOD OF AN INPUT SHGNAL Phillip S. Bengston,Silver Spring, Md, assignor to the United States of America asrepresented by the Secretary of the Navy Original application Mar. 4,1960, Scr. No. 12,876, new Patent No. 3,158,845, dated Nov. 24, 1964.Divided and this application Aug. 31, 1964, Ser. No. 399,955 8 Claims.(Cl. 328-108) This application is a division of my copendingapplication, Serial No. 12,876 filed March 4, 1960 and now US. PatentNo. 3,158,845.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This invention relates to a system for discriminating frequencymodulated signals recorded on magnetic tape and more particularly foreliminating wow and flutter introduced by the recorder playbackapparatus.

This invention also relates to apparatus for obtaining a voltageindicative of the period of a signal.

In addition, this invention relates to a non-linear wave shaping circuitand more particularly to a hyperbolic generator.

Modern te lemetering systems employ magnetic tape to store data in theform of frequency modulated signals. When the stored signals are playedback, variations in the tape speed and length will cause inaccuracies inthe detected signal because the frequency of the potential at the pickupheads will not be indicative of the frequency of the signal originallyrecorded. These variations are generally referred to as wow and flutter.In the past, several systems have been developed to compensate for wowand flutter. Examples of such apparatus are disclosed in US. Patent2,840,800 issued June 24, 1958 to W. H. Chester, US. Patent 2,364,723issued December 12, 1944 to E. W. Kellogg and the application of D. J.Torpy et al., Serial No. 738,582, filed May 28, 1958. The prior systemseither were not sufficiently accurate since they did not have fastresponse or they utilized considerable, complex equipment. Fast responsewas not achieved because low pass filters were usually employed toaverage the signal values. Such filters are unable to respond to respondto variations occurring from cycle to cycle but must relay on variationsover a long period of time to produce an accurate indication of theaverage value of an applied signal.

The present invention obviates the disadvantages of the prior art sinceno filtering is employed. This is made possible because of the uniquecircuits employed to average a voltage over only one cycle and tomeasure the period of an applied potential.

In this novel system, reference and data signals are recorded on amagnetic tape. Any wow and flutter introduced by the recording andplayback equipment will appear on both signals when detected by thepickup heads. Each signal is converted int-o a series of pulses, onepulse being derived for each cycle. The pulses associated with thereference channel are fed into a reference sampler wherein a voltage isderived having an amplitude directly proportional to the period thereof.The output of the reference sampler is gated to a hyperbolic generatorevery time a data signal pulse is produced. The average value of thevoltage fed into this generator is inversely proportional to thefrequency of the reference signal and directly proportional to thefrequency of the data signal. The minimum voltage produced by thehyperbolic generator is indicative of the average voltage each cycle thepulse is produced. The minimum voltage produced by the generator issampled and stored once each data signal cycle. The resulting outputsignal is commensurate with the original signal compensated for wow andflutter.

The unique hyperbolic generator consists of a plurality of exponentialnetworks. Each of the networks has a different potential initiallyapplied thereto which is stored. After the applied potential is removedthe stored voltages will decay exponentially so that at first only oneof the networks will be discharging. After the voltage stored in thefirst network has been sufiiciently reduced, the second network willstart to decay and the process continues from one network to another. Inthis way, an output signal is derived that is directly proportional tothe amplitude of the applied signal and inversely proportional to thefrequency thereof.

The discriminator circuit developed for this use in this inventionutilizes a pulse shaping circuit that produces a pair of pulses forevery cycle of the applied input. One of the pulses actuates a sawtoothgenerator and the other pulse actuates a sampling circuit. The samplingcircuit samples the sawtooth voltage when energized and stores thesampled voltage from one cycle to another. If the sawtooth voltageshould exceed the voltage previously stored, the sampler will follow theincrease.

An object of this invention is to provide a unique compensation systemfor eliminating wow and flutter in magnetic tape playback apparatus.

Another object of this invention is to provide a wow and fluttercompensating system having fast unfiltered response.

A further object of this invention is to provide a unique wave shapingcircuit.

An additional object of this invention is to provide a circuit todetermine the period of a signal.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes between understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 is a block diagram of the system to compensate wow and flutter;

FIG. 2 is a circuit diagram of the reference sample;

FIG. 3 is a circuit diagram of the data sampler; and

FIG. 4 illustrates voltage waveforms that appear in the circuits.

Referring now to the drawings, wherein like reference charactersdesign-ate like or corresponding parts throughout the several views,there is shown in FIG. 1 a block diagram of the entire system. Threeseparate signal channels are recorded on tape 111 which are detected bypickup heads 12, 13 and 14. A reference signal of nominally constantfrequency is picked up by head 12 while heads 13 and 14 pick up signalsthat are frequency modulated in accordance with data. The voltages onheads 12, 13 and 14 are fed to preamplifiers 15, 16 and 17,respectively, which produce rectangular waves in response thereto.

The output of 15, waveform 142 of FIG. 4b, is fed to reference sampler28 which contains pulse shaper 18, sawtooth generator 19 and sampler 21.Shaper 18 generates positive and negative pulses, as shown by 1 44 inFIG. 4d, in response to the leading edge of waveshape 142. The negativepulses actuate sawtooth generator 19 and the positive pulses actuatesampler 21. The sawtooth voltage, FIG. 4e waveshape 146, is sampledeverytime sampler 21 is actuated and stored therein from one cycle tothe next. If the sawtooth voltage exceeds the potential being stored inthe sampler 21, it will follow the increased potential as shown in FIG.4e at 145. The resulting output of the reference sampler 28 is fed inparallel to both data samplers 29 and 3-1.

from the other circuits.

The amplifiers 16 and 17 are connected to data samplers 29 and 31,respectively. Since the construction of both data samplers is identical,it is deemed necessary to only disclose the components in unit 29.

Pulse shaper 22 produces positive and negative pulses, FIG. 4 inresponse to the square wave applied thereto.

- Since the data signals are of higher frequency than the referencesignal, considerably more pulses will be generated by shaper 22 than byshaper 18. One shot mu1tivibrator 23 produces a constant width pulse inresponse to the negative pulse applied thereto. The constant width pulseand the output of sampler 21 are combined in gate 25. The gate is openedwhen the constant width pulse is applied thereto permitting thepotential developed by sampler 21 to be fed to hyperbolic generator 26as shown by the waveform 148, in FIG. 4g. It can be shown that theaverage value of this Waveform is directly proportional to the frequencyof the data signal and inversely proportional to the frequency of thereference signal, i.e. directly proportional to the period of thereference signal. When the gate is closed the generator will produce avoltage that decreases from the output of sampler 21 inversely withrespect to time. Generator 26 is connected to sampler 24 which isactuated by the positive pulses produced by shaper 22. The constructionof sampler 24 is almost identical to that of sampler 21 so the minimumvalue of the hyperbolic signal is stored from one cycle of the datasignal to the next. If the hyperbolic signal is less than the potentialbeing stored in sampler 24, the voltage will be followed as shown inFIG. 411 by waveshape 151.

p The resulting output of the data sampler 29 is a signal varying inamplitude commensurate with the data signal and compensated for wow andflutter.

This can be shown as follows: The output of gate 25 is a pulse ofconstant width and variable amplitude illustrated in FIG. 4g as pulses148. The amplitude is determined by the voltage produced by sampler 21when multivibrator 23 is producing a pulse. The sampler 21 output isdirectly proportional to the period of the reference signal, i.e. kT asshown by waveform 146 of FIG. 42. Hyperbolic generator 26 produces anoutput curve that decreases inversely with time between the pulsesapplied thereto as illustrated by waveform 149 of FIG. 4g. The equationof this curve is given as E r t; where E is the amplitude of the voltageapplied to generator 26; t is the width of the pulse produced bymultivibrator 23; and t is time as measured from the leading edge of thepulse 148. The output of generator 26 decreases inversely with timeuntil the next pulse is initiated by multivibrator 23. The time betweenpulses produced by the multivibrator is equal to the period, t[inversely proportional to frequency] of the data signals. Accordinglythe minimum output produced by generator 26 is E t t Since E =kT E t /t=kT t /t Thus, the voltage sampled by sampler 24 is compensated for likefrequency changes 17 and data sampler 31 where it is combined with theoutput of reference sampler 28. The compensated output of data samplers29 and 31 are connected to any suitable I read out apparatus such as twochannel pen recorder 32.

Reference is now made to FIG. 2 wherein a detailed circuit diagram ofreference sampler 28 is illustrated.

Pulse shaper 18 consists of a diiferentiator comprising capacitor 43 andresistor 42 connected to input terminal 41 and the grid of tube 44. Theanode and cathode of tube 44 are connected to the windings 46 and 47,respectively, of transformer 45. Winding 46 is connected to battery 36through resistor 33 and to ground through condenser 39. These componentsdecouple the pulser Tube 44 is normally maintained beyond cutoff sinceit is connected to battery 37 through resistors 42 and 54. Condenser 55and resistor 54 prevent large variations in the potential of battery 37when tube 44 is pulsed. Winding 48 of transformer 45 is connected tosawtooth generator 19 and sampler 21.

Sawtooth generator 19 contains a pair of triodes 51 and 61. The cathodeof tube 51 is directly coupled to the secondary winding 48 and isconnected to ground by way of resistor 49. Diode 53 and transformer 52connect capacitor 59 to the anode of tube 51. Tube 51 is normallymaintained beyond cutoff by battery 37 which is connected to the gridthrough transformer 52. Diode 53 prevents overshoot when tube 51 ispulsed. The anode of tube 61 is connected directly to battery 36 whilethe grid is connected to a tap on the battery by a biasing circuitcomposed of resistor 57 in parallel with diode 56. The grid and cathodeof tube 61 are interconnected by capacitor 58 and resistor 62.

The voltage on capacitor 59 is coupled directly to sampler 21 whichcontains triodes 63, 66 and 67. The cathode of tube 63 is connecteddirectly to capacitor 59. Diode 64 connects the anode and cathode oftube 63 together. The anode of this tube is also coupled to the grid oftube 66 and capacitor 65. The cathode of triode 66 is connected tobattery 37 by resistor 68, potentiometer 69 and resistor 71 and theanode thereof is supplied by battery 36. The slider of potentiometer 69is connected to the grid of triode 67 The cathode of this tube isconnected to ground by way of resistor 72 and battery 37, and the anodeis also energized by battery 36.

The operation of the reference sampler circuit will now be explained indetail. The rectangular waves 142 of FIG. 4b are obtained from thesignal 141 of FIG. 4a picked up by head 12 of FIG. 1. The rectangularwave is applied to input terminal 41 where it is differentiated bycondenser 43 and resistor 42, producing the spikes of FIG. 4c. Sincetube 44 is biased beyond cutoff only the positive spikes will actuateit. The circuit associated with tube 44 is a triggered blockingoscillator which produces the wavetrain 144 of FIG. 4d across thesecondary of transformer 48. The positive pulse of wavetrain 144actuates tube 63 causing the charge stored on capacitor 59 to betransferred to capacitor 65, thus the voltage on capacitor 59 is sampledwhen the positive pulses are produced by shaper 18. The negative pulsesof wavetrain 144 cause tube 51 to conduct, and discharge the voltage oncapacitor 59. This capacitor is then charged up again as shown bywavetrain 146 of FIG. 42 by battery 36 connected thereto throughcapacitor 58, resistor 57 and diode 56. It is thus seen that tube 51 andcapacitor 59 constitute a bootstrap integrator. When the voltage acrosscapacitor 59 exceeds the voltage stored on capacitor 65, diode 64 willconduct causing the voltage across capacitor 65 to follow any increasethat exceeds the value of the previously stored signal, as shown bywaveform in FIG. 4c. The signal across capacitor 65 is fed into cathodefollower tubes 66 and 67. The slider position of potentiometer 69determines the DC. level of the output appearing at terminal 73. It isapparent from the foregoing that the voltage at terminal 73 will bedirectly proportional to the period of the signal applied to inputterminal 41.

Reference is now made to FIG. 3 wherein a detailed circuit diagram of adata sampler is illustrated. Pulse shaper 22 is of identicalconfiguration to pulse shaper 18, previously described and explained.

One shot multivibrator 23 comprises tubes 93 and 101. Secondary winding83 is connected to the anode of tube 93 by way of condenser 88 and diode89 and to the grid of tube by resistor 114. The anode and grid of tube93 are connected to battery 74 by resistors 91 and 92, respectively,while the resistors 94 and 95 connect the grid and cathode,respectively, of that tube to negative supply 75. Resistor 87 connectsdiode 89 and capacitor 88 to a tap on battery 74 to limit the currentflow through tube 93. The anode of tube 93 is connected to the grid oftube 101 by condenser 96 and the cathodes of these tubes are connectedtogether. The grid and anode of tube 101 are connected to the tap onbattery 74 by resistors 97 and 98, respectively.

The output of the one shot multivibrator 23 is fed from the anode oftube 101 to gate 25 by resistor 99. The gate comprises diode 103connected to the grid of tube 102. Diode 103 is also connected toterminal 73 which is the output terminal of the reference sampler. Thecathode of tube 102 is directly connected to hyperbolic generator 26.

The hyperbolic generator or waveshaper contains a voltage divider havingresistors 115-119 connected between the cathode of tube 102 and ground.Diode 113 connects the cathode of tube 102 to load resistor 121. Thewave shaper comprises a plurality of networks made up of a pair ofseries connected diodes such as 122 and 123. Capacitors 131-134 areconnected between the respective diodes and ground. Each of the networksis connected to a tap on the voltage divider, i.e. between adjacentresistors thereof and resistor 121.

The sampling circuit 24 contains capacitors 104 and 110 which areconnected in parallel with resistor 121 and are also connected to theanode of tube 105. Diode 107 is connected to the cathode of tube 105which is also connected to storage capacitor 111. The cathode of triode105 is connected directly to the grid of tube 106. The output of thedata sampler is taken between cathode resistors 108 and 109 at terminal112.

The operation of the data sampler 24 will now be explained in detail.Square waves obtained from the data signal are applied to terminal 76 ofshaper 22 which produces wave train 147 of FIG. 4 It is noted that thepulses of FIG. 4 are similar to those of FIG. 40.. The number of pulsesin the former is greater than the number in the latter because thefrequency of the data signal is greater than the frequency of thereference signal. Of course, it is understood that wave shapes of thesame type as shown in FIGS. 4a-c are associated with the wavetrain ofFIG. 4 since the apparatus to produce that wavetrain is like that toproduce the wavetrain of FIG. 4d. The positive pulse produced by shaper22 renders tube 105 conductive so the charge stored on capacitors 104and 110 is transferred to capacitor 111. The negative pulse produced bysampler 22 is coupled to the anode of triode 93 and the grid of triode101 and causes tube 101 to cut-off and tube 93 to conduct heavily. Ofcourse, it is understood the positive pulse will not effect tubes 93 and101. Tube 93 will continue to conduct after the pulse has subsided andwill remain conducting until the voltage on the grid of tube 101 hasrisen sufiiciently to render that tube conductive. When tube 101 isconducting a constant voltage is applied to the grid of triode 102. Whenthe pulse is produced the anode voltage of tube 101 is increased to thepotential of the tap on battery 74. This increase in voltage limits theinput of tube 102 to the voltage on terminal '73. Thus the output of thereference sampler is gated through tube 102 to the hyperbolic generator26.

The pulse output of triode 102 is applied to capacitors 131-134 throughdiodes 122-126, respectively. Because of the voltage divider composed ofresistors 115419 the largest voltage is applied to capacitor 131 andsuccessively smaller voltages are applied to the other capacitors andstored by them. When the pulse is removed capacitor 131 will begin todischarge through diode 123 and resistor 121. Initially the othercapacitors will not discharge because the voltage on the cathodes ofdiodes 127-129 will be greater than on the anodes thereof, i.e. equal tothe voltage across capacitor 131. As the voltage across 131 decreases itwill become equal to that stored on capacitor 132 and diode 127 will berendered conductive. This operation continues for the networksassociated with capacitors 133 and 134 so that the output on loadresistor 121 is a hyperbolic function, i.e. the voltage decreasesinversely with time. Of course it is understood that any other suitablefunction can be obtained by the use of this type of network by properlychoosing the component values. Also, the accuracy of such a generatorincreases with the number of networks employed.

The output of hyperbolic generator, FIG. 4g, waveform 149 is fed tosampler 24. The construction and functioning of sampler 24 is similar tothat of sampler 21. However, since a decreasing voltage is applied tosampler 24 the plate capacitor thereof is connected to the input and thestorage or output capacitor is connected to the cathode of triode 105.When the voltage across capacitors 104 and 110 becomes less than thepotential stored on capacitor 111, diode 107 conducts causing thepotential across capacitor 111 to follow any decrease that is less thanthe value of the previously stored signal, as shown by waveform 151 inFIG. 4h. The voltage on the cathode of triode is fed into cathodefollower 106 resulting in a direct voltage varying in amplitude inaccordance with the frequency of the data signal and com-' pensated forwow and flutter introduced by the recording equipment.

Obviously many modifications of the present invention are possible inthe light of the above teachings. It is understood that any desirednumber of channels can be recorded on tape 11. When the number ofchannels is altered, the corresponding number of data samplers and a penrecorder having the appropriate number of output channels must beemployed. If desired, one of the data channels may also contain a timingsignal. Of course the batteries may be replaced by any suitable powersupply.

It should be understood, of course, that the foregoing disclosurerelates to only a preferred embodiment of the invention and thatnumerous modifications or alterations of the invention may be madewithout departing from the spirit and scope of the invention as setforth in the appended claims.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. Apparatus to measure the period of a signal comprising first means toproduce positive and negative pulses for each cycle of a signal appliedthereto, generator means coupled to said first means to initiate asaw-tooth voltage in response to a pulse of one polarity, said sawtoothvoltage having a peak value and sampling means coupled to said firstmeans and to said generation means for sampling the saw-tooth voltagewhen the pulse of the other polarity occur-s, whereby an output producedby said sampling means is indicative of the period of the signal appliedto said first means.

2. The apparatus of claim 1 wherein the positive and negative pulses areof short duration and one occurs immediately after the other. 3. Theapparatus of claim 2 wherein said sampling means includes means to storethe peak value of the sawtooth voltage, and means to follow thesaw-tooth voltage when a previously stored peak value is exceeded.

4. The apparatus of claim 2 wherein said sampling means comprises avacuum tube having an anode, a grid connected to said first means and acathode connected to said generator means, a diode connected between thecathode and anode of said tube, a first capacitor having one sideconnected to the cathode of said tube, and a second capacitor connectedbetween the anode of said tube and the other side of said firstcapacitor.

5. Apparatus to measure the period of a signal comprising first means toproduce a first pulse of one polarity and a second pulse of anotherpolarity for every cycle of a signal applied thereto, generator meanscoupled to said first means to initiate saw-tooth voltage in response tosaid first pulse, and sampling means having an output and coupled tosaid first means and to said generator means for sampling the saw-toothvoltage when said sec- 0nd pulse occurs whereby the output of saidsampling means is indicative of the period of the signal applied to saidfirst means.

6. Apparatus to measure the period of a signal comprising first means toconvert an input signal into a square wave, a differentiator connectedto said first means, a triggered blocking oscillator connected to saiddifferentiator, a saw-tooth generator connected to said oscillator, anda sampling circuit connected to said oscillator and to said generatorwhereby an output of said sampling circuit is proportional to the periodof the input signal applied to said first means.

7. The apparatus of claim 6 wherein said sampling means comprises avacuum tube having an anode, a grid connected to said first means and acathode connected to said generator means, a diode connected between thecathode and anode of said tube, a first capacitor having one sideconnected to the cathode of said tube, and a second capacitor connectedbetween the anode of said tube and the other side of said firstcapacitor.

0 2 8. Apparatus to measure the period of a signal comprising a triggerproducing circuit to produce pulses indicative of the frequency of thesignal applied thereto, a triggered blocking oscillator connected tosaid circuit, a bootstrap integrator connected to said oscillator, and asampler circuit connected to said integrator and said oscillator,whereby an output of said sampler circuit is indicative of the period ofthe signal.

References Cited by the Examiner UNITED STATES PATENTS 2,581,199 1/1952Moe 328182 2,891,154 6/1959 Holmes -a 329107 2,996,665 8/1961 Polen328109 X ARTHUR GAUSS, Primary Examiner.

JOHN HUCKERT, Examiner.

J. ZAZWORSKY, Assistant Examiner.

1. APPARATUS TO MEASURE THE PERIOD OF A SIGNAL COMPRISING FIRST MEANS TOPRODUCE POSITIVE AND NEGATIVE PULSES FOR EACH CYCLE OF A SIGNAL APPLIEDTHERETO, GENERATOR MEANS COUPLED TO SAID FIRST MEANS TO INTITIATE ASAW-TOOTH VOLTAGE IN RESPONSE TO A PULSE OF ONE POLARITY, SAID SAWTOOTHVOLTAGE HAVING A PEAK VALUE AND SAMPLING MEANS COUPLED TO SAID FIRSTMEANS AND TO SAID GENERATION MEANS FOR SAMPLING THE SAW-TOOTH VOLTAGEWHEN THE PULSE OF THE OTHER POLARITY OCCURS, WHEREBY AN OUTPUT PRODUCEDBY SAID SAMPLING MEANS IS INDICATIVE OF THE PERIOD OF THE SIGNAL APPLIEDTO SAID FIRST MEANS.